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At the end of a call for heat, my HP is cooling my house!

I placed a digit thermometer at the furthest register from my furnace/A-coil. It is located on the second floor at the opposite end of the house. When there is a call for heat, the thermometer eventually reads ~7 to ~10 above 68F. When the call for heat cycle ends, the temperature reading slow declines until the output temperature is ~66F (less than ID temp).

Is this a "problem" with all HVAC systems when the fan is set to ON? Is there a fix to this problem?

You have the fan set to "ON" or constant circulation. This means that when there is no call for heat the fan is running at whatever speed it's set to in the constant circulation mode. If the air blowing out of the registers is uncomfortable when there isn't a call for heat check with your contractor to see if the speed of the blower can be set lower for constant circulation. A slower air speed usually will not cause as much discomfort.

Secondly the air that is coming out of the registers is being pulled into the blower from the return ductwork that is connected to your return air registers. "Usually" these return air registers are located near the floor. Cold air sinks and warm air rises. You thermostat (set to 68&#176 is located approximately 5' above these return registers. The air at the floor could be as much as 2&#176; cooler than the air at your thermostat.

This is all assuming that you verified that the digital thermometer you are using is in complete agreement with your thermostat in telling you what the temperature is.

Constant circulation is used for many different reasons in homes around the country. Some people use it to filter the air constantly. Some people use it as a "bandaide" to solve larger differences in temperatures between rooms and floors (a poor man's zoning if you will). Others have their system fan running continuously because they have no idea how to shut it off. I even had a few customers run their fan constantly because they couldn't stand the house being quiet.

You didn't mention why you were measuring temperatures out of the register....... what started this quest for information?

Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

The temperature in that room is lower than the set point on the thermostat. By keeping the fan on, I am trying to maintain a more uniform temperture in that room. That is why I am on this quest.

Is there adequate return air in this room? I'm guessing the room either doesn't have enough return air or large amounts of glass or some other reason it's loosing heat faster on average than the rest of the home.

You should be seeing higher temperature rise when heating with the heat pump running. What are other registers reading as far as temp when the heat is running. Trace the difference referenced to where the registers are connected to the ductwork. This might show you where you're loosing the heat. If you see 20° rise near the furnace and only the 7-10 at the one you're now measuring, the heat is being lost via the ductwork. Insulating the ductwork will solve this problem and possibly bring the room closer to set point.

This isn't necessarily the answer, but something to think about.

Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.